For the root filesystem, on / type xfs (rw,noatime)
For larger partitions (per the cryptoloop doc) the partition itself is
setup via cryptoloop and then XFS ontop of that. Nothing special.

Just to make sure we have all data here - you see the problem only
with loop devices backed by XFS, only with loop devices backed by
raw partitions or with both?

Both.

Any way to trgiger it with just loop but not crypto, etc.

Unfortunately not that I am aware of.. Would trying the kernel with the
patch removed and mounted with -o nobarrier help to show us anything, or?

Actually, yes - seeing what happens if you run plain 2.6.30 or 2.6.31-rc
without the backout patch, but with -o nobarrier would be very
interesting.

Will give it a try with 2.6.31-rc6 and mount all loop devices (both those
backed by XFS and raw partitions) with -o nobarrier to see if the problem
recurs. So far though with that patch up until this next test, there were no
problems.